After creating my own custom weapon in Blender I made it .nif file using Nifscope but when I use Creation kit to assign this model to a weapon it shows me pling in a red rhomb and says: "Marker error". After that instead of a weapon. Inside the CK I get this rhomb again.

Not quite understanding what you're saying, but you might want to try http://tesalliance.org/forums/index.php?/topic/4634-3d-craftworks-importing-weapons-into-skyrim/ on how to import weapons into Skyrim.

I didn't get my mistake either. But the tutorial is great. First I was following a YouTube tutorial which wasn't of a great quality. After looking at the one you linked I got everything working. I think I had problems with textures. Thanks a lot!